‘Oumuamua, Thin Films and Lightsails

byPaul GilsteronOctober 29, 2018

The interstellar object called ‘Oumuamua continues to inspire analysis and speculation. And no wonder. We had limited time to observe it and were unable to obtain a resolved image to find out exactly what it looks like. This morning I want to go through a new paper from Shmuel Bialy and Abraham Loeb (Harvard University) considering the role radiation pressure from the Sun could play on this deep sky wanderer. Let’s also review what we do know about it, which I’ll do with reference to this paper’s introduction, where recent work is discussed. For it seems that each time we look at ‘Oumuamua anew, we find something else to talk about.

Discovered in October of 2017 by the Pan-STARRS survey (Panoramic Survey Telescope and Rapid Response System) in Hawaii, ‘Oumuamua stood out because of its hyperbolic trajectory, flagging it as an interstellar object, the first ever discovered passing through the Solar System. The object’s lightcurve indicated both that it was tumbling and had an aspect ratio of at least 5:1 and perhaps higher, an unusual shape for known asteroids and comets. I won’t provide all the references here, as they’re easily found both in the paper cited below and in other related work.

Image: The track of `Oumuamua as it passed through the inner solar system in late 2017. Credit: Brooks Bays / SOEST Publication Services / Univ. of Hawaii.

What stood out in 2018 was detection of non-gravitational acceleration in ‘Oumuamua’s motion, which could be consistent with cometary activity, although as we’ve seen in other posts on this site, no such activity has been noted by way of a cometary tail or gas emission and absorption lines. This despite a relatively close approach to the Sun of 0.25 AU. The Micheli et al. paper noting the acceleration was addressed in another 2018 paper from Rafikov et al., which pointed out that torque from any cometary outgassing should have had effects on the object’s spin, but this does not appear in our admittedly limited observations.

What Bialy and Loeb consider in today’s paper is the possibility that solar radiation pressure — imparted by the momentum of photons from the Sun — is responsible for the acceleration, expressed as an excess radial acceleration ∆a ∝ r−2, where r is the distance of ’Oumuamua from the Sun. If so, ‘Oumuamua would of necessity be a thin object with a small mass-to-area ratio — this is required in order to make the radiation pressure effective.

We can work out constraints on the object’s area through its observed magnitude. The paper proceeds to show that a thin sheet roughly 0.3 mm thick and some 20 meters in radius will allow the non-gravitational acceleration computed in the Micheli paper.

I was intrigued enough at this point to ask Dr. Loeb about those dimensions, which vary with albedo (the incident light reflected by a surface). He told me that the 20-meter figure would be the radius if the object is a perfect reflector, though the size would be larger if the value for the albedo is smaller. We do see variations in reflected light as ‘Oumuamua rotates over an eight-hour spin period. Thus, considering the object as a thin surface, we could imagine a conical or hollow cylindrical shape. “You can easily envision that by rotating a curved piece of paper and looking at its net surface area from different viewing angles,” Loeb told me.

So let’s back up a moment. We are asking what properties ‘Oumuamua would have to have if its non-gravitational acceleration is the result of solar radiation pressure. We do not know that solar radiation is the culprit, but if it is, the object would need to be a thin sheet with a width in the range of 0.3 mm. This scenario explains the acceleration but forces the question of what kind of object could have these characteristics. A major problem is that, as mentioned above, there are too many degrees of freedom in our observations to nail down what ‘Oumuamua looks like. We did not have observations sensitive enough to produce a resolved image.

Image: Oumuamua as it appeared using the William Herschel Telescope on the night of October 29. Credit: Queen’s University Belfast/William Herschel Telescope.

We do know that if ‘Oumuamua is accelerating because of solar photons, it must represent what the paper calls ‘a new class of thin interstellar material.’ The researchers note the possibility that such material is naturally produced in the interstellar medium, but go on to consider an artificial origin. Could ‘Oumuamua be debris from a technological civilization, a discarded lightsail?

A fascinating speculation indeed. From the paper:

Considering an artificial origin, one possibility is a lightsail floating in interstellar space as debris from an advanced technological equipment (Loeb 2018). Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative. The lightsail technology might be abundantly used for transportation of cargos between planets (Guillochon & Loeb 2015) or between stars (Lingam & Loeb 2017). In the former case, dynamical ejection from a planetary system could result in space debris of equipment that is not operational any more (Loeb 2018) and is floating at the characteristic speed of stars relative to each other in the Solar neighborhood.

We’re past the stage where we can image ‘Oumuamua with our telescopes and it’s too late to get a mission off to chase it with chemical rockets, which means that the only way we have of pressing the investigation forward is to look for other such objects in the future. But we can look at the properties of thin films to determine whether an object like a lightsail could survive interstellar travel, given encounters with dust and gas between the stars. Returning to the paper:

Collisions with dust grains at high velocities will induce crater formation by melting and evaporation of the target material. Since the typical time between dust collisions is long compared to the solidification time, any molten material will solidify before the next collision occurs, and thus will only cause a deformation of the object’s surface material, not reduction in mass. On the other hand, atoms vaporized through collisions can escape and thus cause a mass ablation.

Bialy and Loeb find that for the mass-to-area ratio they have calculated for a thin-film ‘Oumuamua, the object could travel through much of the galaxy before losing a significant fraction of its mass. Collisions with gas particles in the interstellar medium as well as the stresses of centrifugal and tidal forces are also considered. None of these present problems for the object’s survival until we reach a maximal travel distance in the range of 10 kpc (well over 32,000 light years). Earth is approximately 25,000 light years from the center of the galaxy.

Image: This is Fig. 1 from the paper. Caption: The maximum allowed travel distance through the interstellar medium (ISM), as a function of (m/A). The blue and red lines are limitations obtained by slow-down due to gas accumulation, and vaporization by dust-collisions, respectively. The plotted results are for a mean ISM proton density of (n) ∼ 1 cm−3. All lines scale as 1/(n). The dashed magenta line is our constraint on ’Oumuamua based on its excess acceleration. The Solar Galactrocentric distance is also indicated. Credit: Bialy & Loeb.

Avi Loeb has recently written about how we might find evidence for extraterrestrial civilizations long gone. From that perspective, he considered the possibility that ‘Oumuamua is just such an artifact, and told me this in an email:

With the perspective of my recent essay in Scientific American, `Oumuamua could be defunct sails floating under the influence of gravity and stellar radiation. Similar to debris from ship wrecks floating in the ocean. The alternative is to imagine that `Oumuamua was on a reconnaissance mission. The reason I contemplate the reconnaissance possibility is that the assumption that `Oumumua followed a random orbit requires the production of ~1015 such objects per star in our galaxy. This abundance is up to a hundred million times more than expected from the Solar System, based on a calculation that we did back in 2009. A surprisingly high overabundance, unless `Oumuamua is a targeted probe on a reconnaissance mission and not a member of a random population of objects.

Loeb also made a comment in his email that ties in to what Breakthrough Starshot is attempting to quantify, a series of missions to the same target — hundreds if not thousands of probes — sent swarm-like to ensure that at least one or a few come close to the world under observation. If something like this were happening with ‘Oumuamua, and given that PAN-STARRS barely detected the object at closest approach, we would not know about any of its fellow probes.

Addendum: Just as I was publishing this I learned of a paper by Eric Mamajek (University of Rochester), who notes that ‘Oumuamua appears to have originated at the Local Standard of Rest (LSR), which is the galactic frame of reference. Quoting Mamajek: “Compared to the LSR, ‘Oumuamua has negligible radial and vertical Galactic motion…” According to Dr. Loeb in a subsequent email, less than one star in 500 is at that frame of reference to the same precision. (The Mamajek paper, “Kinematics of the Interstellar Vagabond 1I/’Oumuamua (A/2017 U1),” can be found here in preprint form).

It’s an interesting point. If ‘Oumuamua turned out to be artificial, would there be an advantage in such a position? Perhaps so, for as Loeb goes on to say in his email:

I view a sail (like `Oumuamua) floating in interstellar space with stars (like the Sun) running into it as if it were a buoy floating on the ocean surface with boats colliding with it. An artificial origin would naturally place floating sails at the LSR, perhaps as relay stations.

Another item of interest:

Trilling et al., “Spitzer observations of `Oumuamua and `Oumuamua’s density and shape,” as presented at the American Astronomical Society DPS meeting #50, finds no detection of thermal emission from `Oumumua, implying that it must be very reflective or small, which is consistent with the Bialy and Loeb paper. The abstract of the Trilling paper is here.

Thanks, Hamilton1.
The two reports look like excellent studies. At first scan, it looks like there are still things to examine though. For example, if ‘Oumuamua is considered representative of extra-solar objects in its debris trail, then the idea that extra solar meteorites showing up, say in Greenland, is in real trouble. But if other exo-objects behave like local ones, but with distinctly different isotope ratios, then maybe scientific investigation will still have a toe-hold somewhere.

Every one here seems t speculate that if this was a probe it is currently sailing away. I believe that if it is indeed a probe it detached from the solar sail near perihelion causing the solar sail to tumble. The probe itself could be of the size of a smartphone and has probably further decelerated after detaching from the sail. It is now perhaps heading towards earth. But given the size of the probe it is unlikely we will be capable of detecting it.

While I doubt that ‘Oumuamua is artificial (although I would love to be wrong!), a simple experiment could possibly provide positive results:

If radio and/or laser signals (of the type that have been proposed for triggering a response from a Bracewell probe) were beamed at ‘Oumuamua (and/or into the area of space where a small, separated probe might be), perhaps we would get an answer. While I doubt whether any response would be forthcoming, we just don’t know, and such an experiment would not be prohibitively expensive to conduct.

A scientific paper led by two researchers at Harvard University made a splash this week by claiming that a cigar-shaped rock zooming through our solar system may have been sent by aliens.

The researchers noted in a pre-print of the article that it was an “exotic scenario,” but that “Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.”

“Currently there is an unexplained phenomena, namely, the excess acceleration of Oumuamua, which we show may be explained by the force of radiation pressure from the sun,” co-author and Harvard astrophysicist Shmuel Bialy told AFP via email Tuesday.

“However this requires the body to have a very large surface and be very thin, which is not encountered in nature.”

Their suggestion of an alien force at work went viral.

But other astronomy experts aren’t buying it.

“Like most scientists, I would love there to be convincing evidence of alien life, but this isn’t it,” said Alan Fitzsimmons, an astrophysicist at Queens University, Belfast.

“It has already been shown that its observed characteristics are consistent with a comet-like body ejected from another star system,” he told AFP.

“And some of the arguments in this study are based on numbers with large uncertainties.”

…

Katie Mack, a well-known astrophysicist at North Carolina State, also took issue with the alien hype.

“The thing you have to understand is: scientists are perfectly happy to publish an outlandish idea if it has even the tiniest sliver of a chance of not being wrong,” she wrote on Twitter.

“But until every other possibility has been exhausted dozen times over, even the authors probably don’t believe it.”

Asked if he believed the hypothesis he put forward, Bialy told AFP:

“I wouldn’t say I ‘believe’ it is sent by aliens, as I am a scientist, and not a believer, I rely on evidence to put forward possible physical explanation for observed phenomena.”

The other co-author, Avi Loeb, chairman of Harvard’s astronomy department, told NBC News humanity may never know more about the mysterious object, since it has traveled far away and isn’t heading back.

“It is impossible to guess the purpose behind Oumuamua without more data,” Loeb was quoted as saying.

Their paper was accepted for publication in the Astrophysical Journal Letters, and will appear on November 12.

Oumuamua, Hawaiian for “messenger” or “scout,” was first viewed by telescopes in October 2017.

The alien rock is about 1,300 feet long (400 meters) long, and only about 130 feet wide.

Is ‘Oumuamua, the weird object from interstellar space that barreled
through our solar system late last year, an alien spaceship?

Spoiler: No.

Well, almost certainly no. I can’t say unequivocally that it’s not, but I can say that I would bet a lot of money against it.

[tl;dr for impatient people: Some of the behavior of ‘Oumuamua fits with a ship, but more importantly some important behaviors do not. My conclusion is that it’s something natural. It’s weird — after all, it’s been traveling through the galaxy for thousands of years at least — but natural.]

This article does make clear why even Harvard professors would write a paper speculating about Oumuamua being a light sail star probe. It was not some arbitrary idea they through out there for the heck of it. Like Tabby’s Star, both celestial objects show, if nothing else, that humanity still has a LOT to learn about the real Universe. We are only just beginning to be less clueless about existence.

The object we call ‘Oumuamua entered our solar system a year ago and quickly left again. It’s a very puzzling object. Could it be an alien lightsail? A new paper from Harvard discusses that possibility.

One or more astronomers have predicted (it’s also covered somewhere here on Centauri Dreams, in an article and/or in one or more comments under one) that interstellar objects–natural ones–like ‘Oumuamua probably pass through our Solar System quite often (I forget the suggested figure–perhaps one per year?), and this raises a question:

If many years or decades pass *without* another such object being detected traversing the Solar System (or passing close enough to be detected, assuming that future Earth- or space-based instruments are sharp-eyed enough to do so), how might that affect astronomers’ and SETI/SETA researchers’ positions on whether ‘Oumuamua was likely either artificial, or a rarely-detectable natural object? Now:

The current view seems to be that there are many, many asteroids and comets wandering between the stars. This seems to be a sensible view, considering how the massive gas giant (and even the somewhat less massive ice giant) planets, which are common possessions of stars, can eject such objects from their stellar systems, and how distantly-orbiting Oort Cloud objects around stars can be “perturbed away” by passing stars (some–the comets that we see–are perturbed inward by such gravitational interactions), but:

With interstellar space being so much more voluminous than the space within stellar systems (especially in our region of the Galaxy, where the stars are farther apart than in the spiral arms, the globular clusters, and the core region), is the population density of “‘Oumuamuaoid objects” high enough that we can expect them to ‘visit’ us often? If not, it would appear that the case for ‘Oumuamua possibly being artificial is stronger, although without inspecting it via a probe, it’s possible–which I personally believe is more likely–that we just had a lucky detection of a (possibly relatively rare [thinly-spread, population-wise]) natural object. Also:

There is nothing wrong with engaging in belief in science; in situations where there is a paucity of obtainable data (which is the case here, unless/until a probe is–or can be–dispatched to ‘Oumuamua), it is sometimes all that we have to go on. Even in the everyday conduct of science, although they may seldom think of it, scientists make observations because they believe that they can learn something, or conduct experiments (and/or make observations–in astronomy, for example) because they believe that they can either confirm or falsify theories (if they didn’t believe that it was possible, they wouldn’t bother to do these things).

Astronomers (I do not recall who exactly at the moment) said after Oumuamua that there should be between one to ten interstellar objects passing through our Sol system each year. Now granted they are still hard to detect so we may have missed them, but so far it has been over one year since Oumuamua was found and there have been no reports of another interstellar visitor.

So either someone was a bit too optimistic in their calculations (or strong guesses?) or the Universe once again is not behaving the way humans want it to. And since we do not know when another Oumuamua is going to pass our way again, this is all the more reason to get a space probe to it ASAP – and yes, it can be done.

Oumuamua is similar to Tabby’s Star in the sense that it is a celestial object that is not entirely explainable by current science. Perhaps they are mysteries only in the sense that we are unacquainted with such objects, but they warrant further investigation all the same.

One to ten interstellar interlopers “just passin’ thru” per year–Thank You for reproducing that estimate! They could even be right (or even be under-estimating the actual annual numbers of such visitors), and:

The variations in the size, “actual reflectivity per square meter” (in visible, infrared, and perhaps also ultraviolet light), rotational motion(s), viewing direction with respect to the Sun, and perihelion *and* perigee distances of each interstellar interloper could easily conspire–in many different combinations of these factors–to make them utterly undetectable by us. Asteroid and meteoroid population versus size statistics indicate that as their sizes decrease, their numbers rapidly increase. If this also applies to interstellar ones, there could be tens, hundreds, or even thousands (or more) of them passing by us–although as far as is known, no interstellar meteors have been detected optically or photographically…which would seem to suggest that interstellar meteoroids aren’t so common (the sheer vastness of interstellar space could easily account for this). In addition:

Like Rama, these interstellar visitors don’t seem to “work according to our textbooks,” and I agree that our only sure bet is to send a probe to the one that we *know* exists. Without being prohibitively expensive, a Falcon Heavy- (or perhaps even a Falcon 9 Block 5)-launched ‘Oumuamua probe could be dispatched to it. The spacecraft could utilize a Jupiter-Sun (or perhaps a Saturn-Jupiter-Sun) gravity boost trajectory, with a “Sun-diver” solar sail module (a parachute-type solar sail might be employed, without needing an occulter to emerge from behind at perihelion) being added if the mission’s “velocity budget” required it. As well:

‘Oumuamua’s utter lack (that we could detect) of cometary-type outgassing, coupled with its anomalous post-perihelion acceleration (slower-than-Keplerian deceleration, actually) and its apparent lack of sunlight pressure-induced rotational change, make it well worth examining at close range, even if it’s “only” a natural object (which is where my money is). We’ve never seen or taken closeup measurements of such an ancient, apparently tholins-coated, thoroughly Galactic Cosmic Rays (and possibly also gamma rays)-bombarded object, much less one from another stellar system. If ‘Oumuamua has impact craters, those scars would also offer clues about other objects that drift between the stars. However successful agitation for new probe missions is carried out (lobbying the astronomical community and NASA, I’d suppose, at least for starters), there’s no better time than the present to begin such agitation!

It’s the combination of its shape and its trajectory (passing the Earth from behind and below the elliptic plane) that stand out. If this object were almost-spherical then one would just say another random interstellar rock passing close to Earth. On the other hand if this cigar shaped object had different trajectory like several AU away from the Earth then one would accept it as it is. Both scenarios don’t show anything too interesting to point to ET.

Anyway, crying wolf once in a long time isn’t always bad (maybe in this case), it reminds people the big bad wolf is somewhere out there but it’s unknown when it’ll come or it’ll eventually show up here in the far future. The other alternative is that wolf doesn’t exist hence it’s a waste of time to search for one.

The Harvard professors were playing out on paper a what-if scenario. They were not saying that Oumuamua was an alien vessel with a solar sail. They were just pointing out certain features and behaviors that seem to imply such. It was the sensationalist-driven media and the over-imaginative general public that went to town with this idea.

This happened once with a scientists working on the Large Hadron Collider. A reporter was talking to him about quantum physics and asked him if it were true that objects at the quantum level can pass through solid objects and if somehow the LHC will create this scenario in the macroscopic world. The scientist, being a scientist and not fully appreciating how a reporter might interpret things, answered in science-speak that there was an incredibly small chance of such an action happening. The reporter naturally wrote that LHC scientists think that the collider could cause objects in our world to move through solid walls as if they were not there. Sigh.

Loeb does himself no favors. I accidentally heard an interview he gave on this paper and when pressed hard on his conclusion he retreated to saying “it’s a credible journal” and “it was peer reviewed” (my quotes are likely not exact since I’m going from memory). I was unimpressed with his, IMO, unprofessional stance throughout the interview, not just those bits. Sorry, I have no reference for this interview since I tuned across it by chance and took no notes of the particulars (nor could I at the time).

Re-reading this article (and reading the other ones that are linked-to above), two hypotheses came to mind:

[1] If ‘Oumuamua is natural (I’d wager much more that it is than that it isn’t), perhaps it’s a “dust bunny” of sorts. Dust is common in space, and it can also vacuum-weld together. Maybe it’s a slowly-spinning “dust bunny,” which was formed into a thin sheet by the centrifugal force? Random larger gaps in it, or areas of brighter or darker (or perhaps thicker and more reflective) dust, might account for the albedo changes that were interpreted as tumbling motion in an elongated, solid object (if it’s “coning” or tumbling, such a rotating “dust bunny’s” light curve might also look that way). Also, regarding observed albedo changes:

[2] If ‘Oumuamua is artificial–a rotating lightsail–and has variable-albedo steering panels (like JAXA’s IKAROS interplanetary solar sail, which is spin-rigidized & stabilized), the alternately “presented and (totally or partially) hidden” (depending on the sail’s exact configuration and/or condition) variable-albedo panels might account for the observed albedo changes, especially if the sail is “coning” or tumbling. If the sail is a derelict (perhaps a damaged one; missing pieces would affect its light curve if it’s rotating), even inactive variable-albedo steering panels would cause changes in its observed albedo.

Because of the Spitzer non-detection, if it were a dust bunny or were made of naturally formed aerogel, as Bill Gray suggested, it would have to be made of very REFLECTIVE dust or aerogel, of which NO natural examples have EVER been found.

The comments are the interesting part, it should be studied to see what the general population understands about such things. In the future this type of person and their reasoning is how most uninformed people are going to learn and understand what civilization may be dealing with. If we do come in contact with ET most people are going to be shocked and scared by the unfamiliarity of something that will most likely be completely different from anything we have ever imagined.

More plausible than the light sail would be a monolith-shaped structure like TMA-1 from “2001 a Space Odyssey”, which would be susceptible to light pressure and, if tumbling, would produce the high-amplitude light curve. It is difficult though to understand how this could be a natural object and how it could come to visit us.

In November 2017, scientists pointed NASA’s Spitzer Space Telescope toward the object known as ‘Oumuamua—the first known interstellar object to visit our solar system. The infrared Spitzer was one of many telescopes pointed at ‘Oumuamua in the weeks after its discovery that October.

‘Oumuamua was too faint for Spitzer to detect when it looked more than two months after the object’s closest approach to Earth in early September. However, the “non-detection” puts a new limit on how large the strange object can be.

The results are reported in a new study published today in the Astronomical Journal and coauthored by scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Oumuamua is a sail, it’s a sheet of natural plastic, aka “Tholin” formed on the surface of some outer planet,

Carl Sagan famously described the reddish gunk that accumulates on the outer planets, as Tholins aka “star tar”. It’s a polymer of methane and ammonia-based compounds that have been fused by UV radiation.

The comments saying that it is a planet tar, dust bunny, etc. doesnt fit the high reflectivity and also approaching the Sun these materials will start to heat up and will start to produce particles or gases that at least will change the spin and the tumbling period of the object which in this case wasn’t observed. The current observation is more towards a solid object with extreme proportion, diameter/length to thickness more than 10.
Too agree that sending a probe chasing it will give more accurate observations what it is , what if human kind never spot another Oumuamua?

I believe trying to get to ‘Oumuamua will induce a wonderful technological renaissance and may ultimately be our first foray towards, or into, interstellar space. Those are the things I think about when I think of ‘Oumuamua, and I’m not a scientist, just a regular citizen, but admittedly I don’t really consume much mainstream media. Thanks for your blog, it’s been a fascinating read and I’ve only scratched the surface.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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